Without a good base layer, the rest of your outfit isn&&';;t able to function as designed. As the first barrier between your skin and the elements; it&&';;s critical a good baselayer be warm, comfortable, and moisture-wicking. Just like the Pant, the Pyrex Extreme shirt is in a class of its own in a durable, well cut upper body base layer. The same proprietary GRIDtech 4-way stretch fabric works as a second skin, providing additional warmth without weight and maximum breathability. Polyester/Spandex Blend Grid Fleece for Added Warmth 4-Way Stretch Polyester, Highly Breathable, Moisture Wicking Fabric Flat-Lock Seams Mid/Late-Season Base Layer